THRIFT-214. cpp: Qualify another cast

r722337 only fixed one occurrence of this issue.
Not sure why this one hasn't been a problem before,
but it is breaking compilation on Mac OS 10.6.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@810150 13f79535-47bb-0310-9956-ffa450edef68
1 file changed